1. Field of the Invention
The present invention relates to a light-emitting device that is able to realize a high luminous efficiency by emitting a light generated in a light-emitting element with a high efficiency, and a method for manufacturing the same.
2. Description of the Related Art
Refractive indices of compound semiconductors constituting light-emitting devices are so high that much luminous energy is lost by reflection at the surface and interface of most of the light-emitting devices and the like. Therefore, it is difficult to emit the light generated within the element to outside. For example, since the compound semiconductor such as gallium phosphide (GaP) has a refractive index of as high as about 3.5, and only about 19% of the light generated in the semiconductor is emitted due to total reflection. For solving this problem, a monolayer with a refractive index of about 1.5 is formed on the surface of the light-emitting element or the like as an antireflective film. However, luminous efficiency is yet insufficient in this type of light-emitting device due to a relatively large refractive index difference between the light-emitting surface and the monolayer film.
It is contemplated to increase transmittance by forming ordered structures with a size of several nanometers on the surface of the light-emitting element, in order to enhance the light emission efficiency (see non-patent document 1 below). However, since it is almost the limit of photolithography even by using a latest eximer laser to form the ordered structure having a refraction preventive effect with a size of several nanometers, the structure should be formed by an electron beam and etching. Consequently, this method is not practically applicable due to its high cost and poor throughput. Moreover, the process for forming the nanometer size of ordered structures has small process window.
Another method known in the art for roughening the light-emitting surface is to treat the surface with hydrochloric acid, sulfuric acid, hydrogen peroxide or a mixed solution thereof (see patent documents 1 and 2 below). However, some crystal faces can be roughened while the other crystal surface cannot be roughened by this method by being affected by crystal orientation of the substrate. Accordingly, the light-emitting surface cannot be always roughened by this some limitations for increasing light emission efficiency.    [Non-patent document 1] Applied Physics Letters, 142, vol. 78, 2001; Jpn. J. Appl. Phys., L735, vol. 139, 2000    [Patent document 1] Japanese Patent Application Laid-open No. 2000-299494    [Patent document 2] Japanese Patent Application Laid-open No. 4-354382